Audio frequency amplifier



-May 12, 1942. c. 5. R007 7 2,282,383

AUDIO FREQQENGY AMPLIFIER Filed Sept. 13, 1939 Contact [3 at top of resistance /0.

Gain

I 30 I00 200 v 1000 2500 4000 10000 Frequency Fig. 4.

Contact 13 low on resistance 10.

Gain

l l I l l 30 I00 400 I000 4000 [0000 Frequency Inventor" Charles 5. Roof,

His Attorney.

Patented lVlay 12, 1942 2,282,383 sumo msousncr AMPLIFIER Charles S. Root, Bridgeport, Conm, assignor to General Electricv Company, a corporation of New York Application September 13, 1939, Serial No. 291,859

16 Claims.

My invention relates to audio frequency amplifiers and particularly to such amplifiers adapted for use in radio receivers for ordinary home reception.

It has for one of its objects to efiect certain improvements therein by way of increasing the apparent output volume of such amplifiers as well as improving their tonal response.

Degeneration is commonly employed in such amplifiers for reduction of hum and distortion, and for other reasons. I have in my patent application Serial No. 221,319, entitled Feedback circuit, filed July 26, 1938, disclosed such a system which has been found to be advantageous.

My present invention has for its object to provide certain improvements in such systems whereby the response of the amplifier at desirable high frequencies may be increased by reducing the degeneration whereas at very high frequencies the response may be reduced by restoring the degeneration thereby to reduce the reproduction of high frequency noise currents and whereby at the same time, the apparent oral response of the amplifier when adjusted for maximum volume of output is increased.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further ob- Jects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawing in which Fig. 1 represents an embodiment of my invention; Fig. 2 represents a modification thereof; and Figs. 3 and 4 represent certain characteristics pertaining to my invention.

Referring to Fig. 1 of the drawing, I have shown therein an audio amplifier comprising electron discharge 'devices I and 2 which are connected to amplify the voltage which is detected by detector 3 in a radio receiver and to supply said voltage toa loud speaker 4. The detector 3 is the ordinary diode detector of the conventional broadcast receiver adapted for home reception and is connected vin series with a load resistance 5 and the secondary winding of a transformer 6, which may be tuned to the lows the envelope of the rectified carrier wave and thereby reproduces the audio frequency signal potential. This potential is supplied through a coupling condenser 8 between diagonally opposite corners of a bridge comprising resistances 9, II), II and I2, the purpose of which later will be described. The resistance III is the ordinary volume control potentiometer of the receiver having a variable contact I3, which is connected through a coupling condenser I4 to the grid of the electron discharge device I, the latter of which is connected through grid resistor l5 to the cathode of the discharge device. Anode potential, for operation ofdischarge devices I and 2, is supplied from any suitable source represented by terminals I6, one of these terminals being connected through the primary winding of the output transformer H to the anode of discharge device 2 and through resistances I8 and I9 to the anode of discharge device I. It is also supplied through the resistance l8 to the screen grid of the discharge device 2.

Condenser 20 is connected between the anode and cathode of discharge device I, and condenser 2| is connected between the anode and cathode of discharge device 2 thereby to by-pass any currents of radio frequency and thereby prevent their transmission through the system. Condenser 2| ordinarily is so proportioned relative to the inductance of the transformer I! as to produce a condition of resonance in the range of frequencies where the loud speaker 4 has its maximum response, this range of frequencies ordinarily being between about 2,000 or 2,500 and 5,000 cycles.

Voltage from the voice coil 22 of the loud speaker 4 is supplied through conductors 23 and 24 to the opposite terminals of resistances I I and I 2 of the bridge, these terminals, of course, comprising a second pair of diagonally opposite terminals of the bridge. The voltage from conductor 24 is of course supplied through resistance 9 to point 26 in phase to produce regeneration while that from conductor 23 is supplied through resistance I0 to point 28 in phase to produce degeneration due to feedback from the voice coil to the input of device I.

The point 25 between resistances II and I2 is connected to the cathode of the amplifier I, and the resistances 9. III, II and I2 are so chosen relative to each other that the point 26 is at the same potential with respect to voltages supplied from the output to the input by conductors 23 and 24, as is the point25 between resistances II and I2. This means that when the contact I3 is at the upper end of the volume control resistance I there is novoltage supplied from the output of the amplifier to the input thereof, the regenerative and degenerative voltages neutralizing each other. On the other hand, when the contact I3 is at th ower end of resistance I0 then the entire voltage of resistance I2 is supplied between the gridand cathode of amplifier I in degenerative phase. potentiometer contact I3 is moved downward, the amplifier becomes more and more degenerative.

Of course, if desired, the bridge may be so balanced, by proper proportioningof the arms thereof, that when the contact is at the extreme upper point on the potentiometer III, regenerative voltage is supplied to the grid of amplifier I and when it is moved down, this regeneratiy voltage is first reduced to zero and then a degenerative voltage from resistor I2 is supplied to the input of the amplifier.

In receivers where a large apparent response is required, it is desirable, as previously described, to tune the output from amplifier 2 by means of condenser 2I and the inductance of transformer I1 to a frequency in the neighborhood of the frequency at whch the loud speaker has maximum response, this frequencybeing ordinarily between 2,000 or 2,500 and 4,000 kilocycles. with the amplifier so adjusted, were contact I8 moved to the upper end of potentiometer I0 thereby to remove all degeneration, the receiver would have a marked maximum, or peak, of response in the above range of frequencies. If contact I3 be then moved downward, thereby applying degen- 1 eration, if nothing further were done, the efiect of the degeneration would be greatly to reduce or even eliminate entirely this maximum of response.

This is pndesirable, however, since a maximum of response at high frequencies is necessary at low settings of the volume control potentiometer to compensate for high frequency attentuation, or side band cutting, which takes place in the previous tuned amplifiers, thereby impairing the tonal quality of the output signal. In fact, when contact I3 is at a lower position it is preferred to have the high frequency peak of response at an even higher frequency such as 4,500 cycles thereby further to improve the tonal response. Also it sometimes occurs that by reason of phase shift at high frequencies produced by condensers 20 and 2I, regeneration occurs at frequencies above 10,000 cycles, thereby producing a peak of response in that range of frequencies. This is.

not desirable because it tends to accentuate the reception of noise currents.

In accordance with my invention, means are provided whereby the degeneration of the amphfier is greatly reduced or removed, or the amplifier is made regenerative, in the range of frequencies between 2,000 or 2,500 and 5,000 cycles,

while the amplifier remains highly degenerative at frequencies both above and below this range of frequencies. This is accomplished by the simple addition to the circuit already described of condensers 21 and 28 and resistance 29.

Condenser 28 and resistance 29 are connected between conductors 23 and 28 and the point benetwork, to supply voltage from the loud speaker voice coil to the resistance I2 between the grid and cathode of, the amplifier I in'addition to that supplied to resistance I2 through resistance II. small so that for 400 cycles and frequencies lower than that these condensers ha a very high In other words, as the V impedance relative to resistance II and thus the path comprising these elements at those frequencies has substantially no effect upon the operationof the receiver, all of the. degenerative voltage being supplied to resistance I2 through resistance II. Maximum degeneration occurs at these frequencies. At higher frequencies, however, the portion of the voltage supplied from the conductor 24 to resistance I2 through condenser's 21 and 28 rapidly riseswith frequency as the reactance of these condensers becomes smaller and approaches the resistance of resistor H, these voltages being firstiattended with very great phase shift thereby tending to advance the phase of the total voltage on resistance I2 from the degenerative phase relation and thus reduce Ihe degeneration of the amplifier. At the same time a phase shift in the opposite direction is caused by condensers 20 and 2| in the amplifier, these condensers tending to retard or reduce the advance in phase of voltage in resistance I2. This opposite phase s'hift' however, is, at these frequencies below the frequency at which the response of the amplifier is maximum, fairly small. The result is that at a frequency where the reactance of condensers 28 and 21 each become of the same order of size as the resistance'of resistor II, the phase of voltage on resistance I2 has its maximum advance or its least degenerative efiect. It may even be regenerative although this is not .usually desirable. By proper choice of condensers 28, 21 and resistors 29 and I2 this may 7 7 1y simulated or even exceeded, and this peak is thus eifective to improve tonal response by compensating for attenuation of these desirable frequencies in the tuned circuits of the radio or intermediate frequency amplifiers.

At higher frequencies the advance in phase of voltage on resistance I2 caused by the network 28, 29, 21, I2 is gradually reduced as the reactances of condensers 28 and 21 become smaller, thus gradually restoring the degenerative condition. This effect is aided by the increasing retardation of phase produced by condensers 20 and 2I. Thus, the amplifier is sufiiciently degenerative at frequencies above 10,000 cycles to prevent any peak of response at those frequencies. In fact at very high frequencies the re-' tardation of phase may be so great as to exceed the phase position for maximum degeneration and degeneration is reduced because of retardation rather than advance of phase. This may go so far that the system may become regenerative. If this occurs, however, it occurs at frequencies so very high that the audible effect is negligible. In one receiver in which resistance I I was of a value of 1,500 ohms; resistance I2, 270 ohms; re-

sistance 29, 220 ohms; condenser 28, 0.1 micro- Condensers 21 and 28 are'sufiiciently 2,282,883 curred when operating at low settings of the potentiometer for normal volume. The output was reduced by degeneration in the ratio of to 1 at 400 cycles," 3 to 1 at 4,500 cycles and-2 to 1 at t limitation upon my invention as these values may be varied through wide ranges while securing the results my invention is contemplated to se,-

' cure.

In another equipment embodying my invention in which resistance II had a value of 6,800 ohms; resistance I2, 4,700 ohms; resistance 29,

220 ohms; condenser 28, .1 microfarad; condenser 21, .01 microfarad, the response at 4,500 cycles was four times the intensity at 400 cycles at a similar low setting of the volume control. The output was reduced in a ratio of 15 to 1 by degeneration at 400 cycles, 3 to l at 4,500 cycles and 2 to 1 at 10,000 cycles;

This operation, with a peak of response produced at the frequency where such peak is desired and produced by reduced degeneration rather than regeneration so that degeneration and reduction of distortion occur at all frequencies very noticeably improves the quality of reproduction secured, such reproduction being substantially as satisfactory as that produced by receivers with a push-pull output stage in clarity of response and freedom from distortion.

The network comprising elements 21, 28 and 29 mayoperate to disturb the balance of the bridge 9, III, II, I2 when the contact I3 is at the upper terminal of resistance Ill. The amplifier may then be regenerative at high frequencies by reason of this network 21, .28, 29. This eil'ect is illustrated in Fig. 3 which shows the gain versus frequency characteristic of a receiver similar to that shown in Fig. 1 with the contact I3 at the upper end of the volume'control resistance. Curve A represents the characteristics secured with elements 21, 28 and 29 removed and curve B represents the characteristic secured with these elements present. The increase in gain produced by the addition of elements 21, 28 and 29 above 2,500 cycles isdue to regeneration produced by these elements. This regeneration occurs, however, with the receiver adjusted for maximum sensitivity such as is necessary when receiving extremely weak signals or remote stations and accordingly such regeneration is desirable to aid reception from such stations and also to give greater apparent maximum volume from strong stations. Fig. 4 shows a similar characteristic of a receiver embodying the invention but with the contact I3 near the lower end of the volume control resistance l0. It will be seen that a peak of response occurs at about 4,000 cycles where the gain is very considerably greater than the gain at 400 cycles and that no peak of response occurs above 4,000 cycles. l

My arrangement possesses the important advantage that the condenser 2| may have such a value that it resonates with the input inductance of transformer I1 at the frequency where celving stations which are sufliciently strong so that the volume control potentiometer is at an intermediate position of resistance I0. Moreover y, the degeneration has the advantage of reducing distortion and hum, and increasing bass response.

Fig. 2 .shows a receiver in which the same character of results is secured in a way which possesses certain advantages. In the arrangement of this figure ground is removed'from the cathode of discharge device I, and the right-hand terminal of resistance I2, as well as the bottom of the voice coil 22 are grounded at 30. Condenser 2I is then connected to ground through resistance 29 and the point between these elements 2| and 29 is connected to the cathode of device I through condenser 21. Thus condenser 2|, resistance 29, condenser 21 and resistance I2 serve the same purpose that condenser 28, resistance 29, condenser 21 and resistance I2 of Fig. 1 serve in that arrangement, this arrangement thus eliminating the necessity for one condenser, namely the condenser 28 of Fig; 1.

This Fig. 2 arrangement has the further ad vantage that the voltage on the anode of discharge device 2 is much greater than that upon the voice coil with the result that the resistance 29 may be made smaller in comparison to the reactance' of condenser 2| than was the case for the corresponding resistance and capacitor in the arrangement of Fig. 1 and thevoltage across it may then remain advanced by 90 degrees all there may be greater degeneration at 10,000 cycles thereby tending to reduce the high frequency noise response.

In one equipment embodyingthis arrangement,

resistance II was of 100 ohms; resistance I2, 22 Ohms; resistance 29, 150 ohms; condenser 21, 0.1 microfarad and with such constants the response at 4,500 cycles was 1.6 greater than the response at 400 cycles.

' with that across resistor I2 instead of out of phase the loud speaker has its maximum response,.

thereby to get maximum volume in response to weak stations just as is done in receivers which do-not employ degeneration; and at the same time, by means of the feedback, as described, a peak of respbnse, at a higher frequency than that of maximum response of the loud speaker may as is the voltage from the voice coil. Thus this voltage does not balance out at thetop of the volume control as does the voltage supplied to resistance I2 from the voice coil 22. This voltage thus is effective to produce some degeneration when the contact I3 is at the top of the volume control resistance I0 and thus tends to reduce the peak of response between 2,000 and. 5,000 cycles which is. undesirable because it reduces the apparent maximum Volume at maximum volume control position.

This Fig. 2 arrangement'possesses an additional disadvantage with respect to the production of hum when used in connection with phonographs as in radio-phonograph combinations. One of the output conductors from such phonographic equipment is commonly connected to the phonograph chassis and is used as a ground connection for that chassis. In the arrangement of Fig. 1 this connection would be made to point 25 which is, itself, grounded and thus grounds be had improve the to'nal response when re-- I the phonograph chassis. were. the arrangement. of Fig.

may oftransformer receivers, this purpose is this done with 2, however. the-connec-.-

n and is thus supplied to the input of the am.-

plifier and, causes objectionable reproduccoast the loud p aker.

H J 110! i.course,the other terminal of "the phono-- 8 to the audio amplifienthe circuit of the secbel ig opened in any suitable way.

. Fig.= 2 illustrates anadditional feature of my:

" invention in that degenerative voltage may be suppliedfrom thefloutput of amplifier I through resistance u and switch a: to the P int between base compensation condenser 33 and resistance ll, thereby to, reduce the" r of the receiverito low frequencies. C'o monly, inradio I ffejcted by short circuiting'this bass compensation condenser; It

been-found, however,jthat with the arrangement shown, very decided advantages in securing lower low frequency response can be secured. cover-,- the degenerativevoltage so appliedto Mor " inputis, present in varying degrees for all positions of the'volume control connection it but is maximum when this connection is at the lt s a ground would include resistance l2." Acgraphfloutput is connected through condenser a v degenerative in the region of said maximum of a response -thereby to sald amplifier having, inthe. absence of and f5,000 cycles,"and said degenerative voltage being sufficient, when applied,oblectionably to.

reduce the amplification of said amplifier in the range of frequencies where said maximum occurs,

means to prevent said objectionable reduction of amplification in said-mange while said degenerative voltage isfefiective to reduce the gain of the amplifier at'frequencies both above and belowsaid range, and means to' maintain said amplifier reduce distortionin said region. s

4. The combination, in an amplifier having a I maximum ofresponse' at -a certammgn frequency,

high frequency. and means to displace the phase of said voltage from thejrelation "necessary for point of connection of resistance to'the potentiometer "I. This feature of my invention is better described and is claimed in my copending application Serial No. 291,032, filedAugust' 19,

1939, entitled Amplifiers, and assigned toflthe I same assignee as my present application.

While I have shown particular embodiments of my invention it will, of course, be understood that I do not wish to be limited thereto, since different modifications both in thecircuit arrangement and in the instrumentalities employed may be made and I contemplate by the appended claims to cover all such modifications as .fall

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The combination, in an audio amplifier having a peak 'of response at high frequencies, of two feed-back paths from the output to the input of said amplifier, one of said paths supplying voltage in degenerative phase and the other supplying a degenerative voltage displaced in phase with respect. to said last voltageto said input, and means utilizing said lastvoltage to reduce the degeneration of said amplifier in the neighborhood of said peak while maintainingsaid amplifier degenerative at said peak. a

2. The combination, in an audioamplifier for amplifying currents having a range of frequencies extending from the lowest audio frequencies to the neighborhood of 10,000 cycles having a peak of response at high vfrequencies inter mediate in said range, of means to supply voltage within the true spirit and scope of my invention.

said voltagein the range of frequencies where saidpeak of response occurs thereby to reduce the effectiveness of said voltage in producing degeneration at said peak while said voltage remains degenerative at all frequencies in the,

range to be amplified.

3.Incombination an audio amplifier, means to supply voltage from the output of said amplifier to the input thereof in degenerative phase,

maximum "degeneration in the region of frequencies near said certain frequency while main taining said voltage-degenerative thereby-to prevent the degeneration produced by said voltage from objectionably reducing said maximum of response and at the same'time to reduce distortion by degeneration in the neighborhood of said high frequency. 1

5. The combination, in an audio amplifier having a peak of response at high frequencies, two degenerative feed-back'paths from the output to the input of said amplifier, one of said paths supplying voltage in degenerative phase and the other of said paths including a frequency selective network arranged to advance the phase of voltage supplied therethrough to said input in the region of frequencies where said peakof response occurs, thereby to reduce the degeneration of said amplifier in said region while maintaining said amplifier degenerative in said region,

and means to increase the degeneration of said amplifier with frequency in the region offrequencies above said peak of response.

6. In combination, an amplifier having a peak of response at a desired frequency, a resistance connected in the input of said amplifier, means to supply voltage from the output of said amplifier to said resistance in degenerative phase, a capacity connected between the output of said amplifier and one terminal of said resistance to supply voltage through said capacity to said resistance, said resistance and capacity acting as a phase advancing network, thereby to advance the phase of the resultant voltage on said resistance, and means to give said resistance capacity network a maximum of eflect in the region of frequencies where said peak occurs while maintaining the voltage on said resistance vde- I generative at all frequencies in the range to be amplified. I Q

7. In combination, an amplifier having a peak of response at a. desired frequency, a bridge having one pair of diagonally opposite points connected to a source of currents to be amplified and a second pair of diagonally opposite points connected across the output of said amplifier, the input to said amplifier being connected between one terminal of said source and an arm adjacent the opposite terminal of said source, the arm of said bridge intermediate said one terminal of said source and said adjacent arm having de generative voltage thereon, and frequency selecmaximum of response between 2,500

voltage displaced in phase with respect to said degenerative voltage by an amount variable with frequency, said amount being maximum in the range of frequencies where said peak occurs.

8. In combination, an amplifier having a peak of response at a desired frequency, a bridge having one pair of diagonally opposite points connected to a source of currents to be amplified and a, second pair of diagonally opposite points connected across the output of said amplifier, the input to said amplifier being connected between one terminal of said source and an arm adjacent the opposite terminal of said source, the arm of said bridge intermediate said one terminal of said source and said adjacent arm having degenerative voltage thereon, a resistance and a capacity in series across said intermediate arm and a capacity connected between the output of said amplifier and a point between said resistance and first mentioned capacity, said capacities and resistance being proportioned to displace the voltage on said intermediate arm by an amount variable with frequency, said amount being maximum in the region of frequencies in which said peak occurs.

9. In combination, an audio amplifier having degeneration, said amplifier being adjusted to have a maximum of response in the absence of degeneration at a desired high frequency, a manual volume control member, means responsive to movement of the manual volume control member from its adjustment for low volume to its adjustment for high volume to reduce said degeneration, and means to reduce the effect of said degeneration on said maximum of response when said volume control member is adjusted for low volume and to increase the frequency at which said maximum occurs when said volume contro member is adjusted for high volume. I

10. In combination, an audio amplifieghaving degeneration, said amplifier being adjusted in the absence of said degeneration for maximum of response in a desired region of high audio frequencies, a manual volume control member, and means responsive to operation of said volume control member to increase said degeneration as said volume control member is operated to reduce volume, and additional means to reduce said degeneration in said region of frequencies when said volume control'member is adjusted for low volume while maintaining said amplifier degenerative at all frequencies above said region. 11. In combination, an audio amplifier having degeneration, said amplifier being adjusted in the absence of said degeneration for maximum of response in a desired region of high audio frequencies, a manual volume control member, means to reduce said degeneration in said region of frequencies 'when said volume control member is'adjusted for low volume while maintaining said amplifier, degenerative at all frequencies above said region, and means responsive to'movement of the volume control member in the direction of increased volume to reduce said degeneration.

12. In combination, an audio amplifier having degeneration, said amplifier being adjusted in the absence of said degeneration for maximum' of response in a desired region of high audio frequencies, a manual volume control member, means to reduce said degeneration in said region of frequencies when said volume control member is adjusted for low volume while maintaining said amplifier degenerative. at all frequencies above said region, and means responsive to movement of the volume control member in the direction of increased volume to reduce said degeneration and to increase the frequency at which said maximum of responseoccurs.

13. In combination, an audio amplifier having degeneration,. said amplifier being adjusted in the absence of said degeneration for maximum of response in a desired region of high audio frequencies, a manual volume controlmember, means to reduce said degeneration in said region of frequencies when said volume control mem-,

her is adjusted for low volume while maintaining staid amplifier degenerative at all frequenciesabove said region, and means responsive to movement of the volume control member in the direction of increased volume to render saidamplifier regenerative in the region at which said maximum of response occurs thereby to accentuate said maximum of response,

14. In combination, an amplifier having a maximum of response at high frequencies, a resistance in series with the input to said amplifier, a condenser connected in series with the output of said'amplifier and said resistance to transmit to said resistance a voltage degenerative at all frequencies to be ,amplified, said resistance and condenser being proportioned relative to each other to shift the phase of said voltage in the region of said high frequency maximum of response thereby to render said voltage least effective to reduce amplification of said amplifier in said region.

15. In combination, an amplifier having a maximumof response at high frequencies, a resistance in series with the input to said amplifier, a condenser connected in series with the output of said amplifier and said resistance to transmit to said resistance a voltage degenerative at all frequencies to, be amplified, said resistance and condenser being proportioned relative to each other to shift the phase of said voltage in the region of said high frequency maximum of response thereby to render said voltage least effective to reduce amplification of. said amplifier in said region and the impedance of said condenser reducing with frequency above the frequency of said maximum of response thereby to render said amplifier increasingly degenerative as the frequency increases above said maximum of response. I

16. The combination, in an amplifier for amplifying currents having frequencies extending over a wide audio range and having a desired peak of response at a frequency intermediate in said range, of means to supply voltage from the output of said amplifier to the input of said amplifier in degenerative phase at all frequencies tobe amplified, and means to shift the phase of said voltage in the range of frequencies where said peak occurs thereby to reduce degeneration in said amplifier and preserve said peak, said means operating to render said voltage increasingly degenerative'at frequencies increasingly removed from said intermediate frequency at which said peak occurs.

CHARLES S. ROOT 

